The present invention relates generally to heat-curable silicon-containing compositions. More specifically, this invention relates to compositions which cure by way of a platinum-catalyzed reaction of silicon-bonded olefinic hydrocarbon radicals with silicon-bonded hydrogen atoms, wherein the room temperature catalytic activity of the platinum-containing catalyst has been inhibited by the presence of an inhibitor component whose inhibiting action can be overcome by heating.
Organosilicon compositions in which a platinum-containing catalyst is inhibited in its cure-promoting activity at room temperature by the presence of a catalyst inhibitor are well known in the organosilicon art. Examples of compositions having platinum catalyst inhibitors include those containing unsaturated organic compounds; such as ethylenically or aromatically unsaturated amides, U.S. Pat. Nos. 4,337,332; acetylenic compounds, 3 445,420; ethylenically unsaturated isocyanates, 3,882,083; olefinic siloxanes, 3,989,667; and conjugated ene-ynes, 4.465.818 and 4,472,563; other organic compounds such as hydroperoxides, sulfoxides, amines, phosphines, phosphites and nitriles; and various metal salts.
More relevantly, the unsaturated hydrocarbon diester inhibitors of U.S. Pat. No. 4,56,870, such as diallyl or diethyl maleate, and the bis-hydrocarbonoxyalkyl maleate inhibitors of U.S. Pat. No. 4,562,096such as bis-(2-methoxyisopropyl) maleate, have been found to be effective for delaying or preventing the room temperature cure of organosilicon compositions which cure by way of a platinum group metal catalyzed reaction. However, the cure time and/or the cure temperature of these maleate-inhibited compositions are/is undesirably increased by the use of these inhibitors.
This problem of increased cure time and/or cure temperature in an inhibited platinum-catalyzed system is of particular significance for applications where the organosilicon composition is used to rapidly coat a substrate, such as is practiced in the adhesive release coating art.
In the coating arts, such as the paper coating art, the coating composition that is used to coat a substrate should not cure to the extent that its viscosity has increased substantially before it has been applied to the substrate; however, it should rapidly cure thereafter, preferably with only a moderate amount of added energy. This means that the coating compositions preferably should not experience a doubling of its viscosity at ambient temperature for as long as eight hours, but should cure rapidly, at moderately increased temperature, to such an extent that the coated substrate can be further processed.
Furthermore, when the cured coating composition is to be immediately coated with a reactive adhesive, such as an acrylic-based pressure-sensitive adhesive composition, the release coating must be fully cured before the adhesive composition is applied in order to minimize the bonding of the adhesive to the partially cured silicone coating, a phenomenon known as "acrylic weld".
Compositions having improved curing characteristics, and little or no acrylic weld, have been disclosed in U.S. Pat. No. 4,609,574. The improvements of these compositions have been realized by the use of higher alkenyl radicals, such as hexenyl, instead of the usual vinyl radical, as the reactive olefinic hydrocarbon radical. However, even in these improved compositions, the cure rate of the higher alkenyl reaction site has not been fully utilized because the catalyst inhibitor is excessively active at elevated temperatures.
The quest for the ideal platinum catalyst inhibitor in silicon-containing compositions, particularly coating compositions, continues.